Wraparound transmissions are known from the prior art, by way of which an infinitely variable change in the step-up transmission ratio (or step-down transmission ratio) is possible at least within ranges. To this end, two cone pulley pairs are provided which have in each case two cone pulleys. The cone pulleys are oriented in each case with their cone face toward one another, and can be displaced along their common rotational axis relative to one another between a position at a maximum spacing and a position at a minimum spacing. One cone pulley is usually fixed axially and the other cone pulley can be displaced axially. A pulley wedge which is variable is therefore formed between the cone pulleys of a cone pulley pair. By means of a common wraparound means, for example a transmission chain, the two cone pulley pairs are connected to one another in a torque-transmitting manner. The wraparound means has a plurality of deflecting axes, for example by means of a plurality of chain pins in the case of a transmission chain or an infinitely large number in theory of theoretical deflecting axes in the case of a belt. The wraparound means migrates radially to the outside in a cone pulley pair when its cone pulleys are guided toward one another, and the wraparound means migrates radially to the inside in a cone pulley pair when the cone pulley pairs are moved apart from one another. Said movement is as a rule carried out in each case in precisely an opposite manner at the cone pulley pairs in a wraparound transmission, with the result that the tension of the wraparound means remains (virtually) constant, while the spacing between the cone pulley pairs is fixed, and it not being necessary for a deflecting mechanism or tensioning mechanism to be provided for the wraparound means.
A transmission input shaft is fixed rotationally relative to a first cone pulley pair, and a transmission output shaft is fixed rotationally relative to a second cone pulley pair which is connected in a torque-transmitting manner by means of the wraparound means. A transmission ratio can be set depending on the selected spacing of the cone pulleys of a cone pulley pair from one another in relation to the selected spacing of the other cone pulley pair.
A wraparound transmission of this type is known, for example, from DE 100 17 005 A1. In some fields of application, the wraparound transmission is combined with a customary manual transmission with fixed transmission ratio gears, with the result that a greater transmission ratio spread is achieved with a comparatively lower number of fixed transmission ratio gears.
On account of the wraparound means leaving the pulley wedge which is formed between the cone pulleys of a cone pulley pair in a manner which is not tangential with respect to the connection, in particular on account of polygonal running which results from a (usually) finite pitch of a chain, and other dynamic effects during the entry into and the exit from the pulley wedge, and as a consequence of changes in the transmission ratio or as a consequence of rotational non-uniformities and other vibrations, the wraparound means is set in vibration about the wraparound means plane. The wraparound means plane (or vibration plane for short) is the shortest tangential connection of the effective radius which is the set of the cone pulley pairs, that is to say the spacing of the truncated pulley wedge which is formed between the cone pulleys and corresponds to the width of the wraparound means. In this way, the position of the vibration plane is variable with the change in the transmission ratio. The vibration plane is congruent as a rule with the center plane in the running direction of the respective run, that is to say the pulling run (or load run) or the empty run of the wraparound means. In order to reduce said vibrations, sliding rails are used in the prior art, which sliding rails bear against the wraparound means with as little play as possible over as long an extent as possible and thus suppress vibrations and undulations of the wraparound means. Sliding rails of this type are known, for example, from the abovementioned DE 100 17 005 A1 or in a two-piece version from WO 2014/012741 A1.
It is a problem that different temperatures occur during operation and, in particular, relative to the beginning of start up (cold starting). The sliding rail, or at least the sliding faces in the sliding channel which is formed, is/are manufactured from a material which is particularly low-friction. Plastic is suitable, in particular, for this purpose. In contrast, the wraparound means is to be designed for tensile loading and is therefore as a rule manufactured from a different material, in particular from metal. However, the materials have different coefficients of expansion, with the result that jamming can occur between the wraparound means and the sliding rail at low temperatures, whereas the play which is set up to reduce vibrations and undulations of the wraparound means becomes too great at high temperatures.
WO 2007/068229 A1 has disclosed a sliding rail, in which the sliding rail is assembled from at least two separate elements with different materials, the materials having different coefficients of expansion. Here, the elements are arranged in such a way that the overall coefficient of expansion of the sliding rail is reduced. In this way, the change in the play between the sliding faces and the wraparound means over the change in the temperature is reduced.
In order to avoid jamming at low temperatures, the sliding faces are currently set up in such a way that the play is precisely zero at a temperature of at least the sliding faces of minus 40° C. [Celsius]. This leads to a greater play in an operating situation, however, with the result that vibrations and undulations which lead to undesired noise emissions occur increasingly.